1. Field of the Invention
The present invention relates to a scanning electron microscope in which an electron beam is focused by an objective lens producing a magnetic field leaking onto a specimen and in which secondary electrons and other electrons emitted from the specimen are detected. The invention also relates to a method of detecting electrons in this scanning electron microscope.
2. Description of Related Art
In a scanning electron microscope, the geometry of the objective lens is an important factor in determining the resolution of the instrument. To improve the resolution, the aberration coefficient of the objective lens must be reduced. Accordingly, a high-resolution scanning electron microscope is known in which the aberration coefficient is reduced to less than 3 mm, for example, by the use of an objective lens of in-lens or semi-in-lens type in which the magnetic field on the specimen is strengthened.
In the above-described semi-in-lens type objective lens, a single magnetic lens field is formed below the lower end surfaces of the inner and outer polepieces. In this case, the specimen is placed within this lens field, and a high-resolution secondary electron image can be observed.
Furthermore, in an attempt to reduce the effects of objective lens aberration, a retarding method has been put into practical use. In particular, the energy of the primary electron beam is increased, and the beam is introduced into the objective lens range. The beam is decelerated immediately ahead of the specimen. This method has the advantage that the resolution is enhanced further at low accelerating voltages.
In the retarding method described above, it is customary to use a scintillator or microchannel plate placed above the objective lens to detect secondary electrons or back-scattered electrons emitted from the specimen.
Sometimes, a secondary electron detector is placed above deflection coils, which in turn are located above the objective lens. In this case, secondary electrons are deflected by the deflection coils, resulting in a decrease in the detection efficiency.
Where the retarding method is adopted, produced secondary electrons are accelerated by the accelerating field between the specimen to which a negative voltage is applied and surrounding members that are at ground potential. Since the secondary electrons move upward with high energies, it is relatively difficult to direct the secondary electrons toward the detector and detect them within the objective lens. Where a scintillator or microchannel plate is placed near the optical axis between the objective lens and the deflection coil assembly, imaging is hindered, especially at low magnifications. In particular, if the electron beam is deflected through a large angle for low-magnification imaging, the electron beam is scanned beyond the electron passage hole in the microchannel plate or in an aperture plate. Therefore, the beam is cut off by other than the electron beam passage port. As a result, the resulting low-magnification image lacks its peripheral portion and thus consists only of its central portion.
In view of the foregoing, the present invention has been made. It is an object of the present invention to provide a scanning electron microscope which is equipped with a semi-in-lens objective lens and capable of detecting secondary electrons at a high efficiency, the electrons being emitted from a specimen.
A scanning electron microscope in accordance with the present invention has an inner polepiece, an outer polepiece, and an objective lens that form a magnetic lens field leaking onto a surface of a specimen below the lower end surfaces of the polepieces. The objective lens focuses an electron beam onto the specimen. The inner polepiece is provided with a first opening above the lower end surface of the inner polepiece. A detector is mounted outside the inner polepiece to detect secondary electrons passed through the opening. This microscope is characterized in that a negative voltage is applied to the specimen to form a decelerating electric field for decelerating the electron beam near the surface of the specimen and that a conversion electrode is mounted around an electron beam passage within the objective lens. Secondary electrons emitted from the specimen impinge on the conversion electrode. Secondary electrons produced from the conversion electrode are guided to the secondary electron detector via the first opening and detected.
In the present invention, the negative voltage is applied to the specimen to form the electric field that decelerates the electron beam, the field being positioned near the specimen surface. This reduces the effects of the objective lens aberration. The conversion electrode on which secondary electrons emitted from the specimen impinge is mounted around the electron beam passage within the objective lens. Secondary electrons produced from the conversion electrode are guided to the secondary electron detector via the first opening and detected.
Preferably, the conversion electrode is cylindrical and has an inner surface that produces secondary electrons at a high efficiency. It is desirable that the conversion electrode be a single cylindrical electrode. A second opening is preferably formed in the portion of the inner polepiece that faces the first opening. The conversion electrode may be split into upper and lower parts arranged above and below, respectively, the first opening. In this case, it is not necessary to form the first opening in the conversion electrode.
An attracting electrode to which a positive potential is applied to attract secondary electrons may be positioned near the incident surface of the secondary electron detector. A potential between the specimen potential and the potential at the attracting electrode may be applied to the conversion electrode.
Other objects and features of the invention will appear in the course of the description thereof, which follows.